Age of Mies MR20 Cantilever Armchairs  

I have 2 vintage Mies van der Rohe Chrome and Tan Cane Cantilever Armchairs (MR20). I'm trying to figure out their age. One distinct thing I notice is that they have a small connecting tube or rod connecting the arms and legs with one inset hexagonal fastener/screw. However most pictures of this chair show a clamp-like connector. Does anyone know what the difference is between the two models and if this can help me figure out the age and manufacturer of my chairs? Is there anything else I should be looking for? I posted a picture of one just like it on a Web site that says it's a 1950's Thonet version but I haven't seen any other like it. I would really appreciate any help I can get on this.